Regulator circuit



June 29, 1943. R. R. BLAIR REGULATOR CIRCUIT Filed Nov. 26, 1940 /NVEN-OR RR. BLA/R A TTORNEV Patented June 29, 1943 REGULATOR CIRCUIT Royer R. Blair, New York, N. Y.', assignorwto Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of NewYork Application November 26, 19210,!Serial` Noa-367,188

(C1. ynali- 36) 14 Claims.

This invention relates to regulator circuits` and particularly to `regulator circuits, fior governing the gain on signal transmissionV systems.

One object of the invention is to provide a regulator 'circuit having a -pentode tubef'therein that shall control the potential supplied to the screen grid circuitof the pentode tube in an improved manner to control `the pentode tube characteristic.

Another object of the invention is to provide a regulator circuit having a vacuum tube including a screen grid with a cathode heated by alternating current that shall govern the screen grid circuit in a manner to compensate the tube operation for any changes in the potential of the cathode heating circuit.-

A` further object of the inventionl is to Aprovide a regulator circuit having a pentode .tube with a cathode heated by alternating current and a screen grid supplied with regulated Arectified potential from saidsource of alternating current that shall govern the rectied potential supplied to the,y screen grid in armanner to compensate the tube operation for any changes in the Ypotential of the cathode heating current.

In longsignal transmission Ilines it is necessary to provide amplir'iersk or repeaters. at intervals along the-line to-maintain the energy level of the signalsabovea predetermined level. Regulators or `gain control circuits are required at certain of the` repeater stations to hold the energy level ofthe signals substantially constant. The regulators if of the dynamic type aregenerally controlledby a pilot frequency.

According to the present invention a regulator circuit having a pentode `oscillator tube therein is employed to control a feedback amplifier of the type disclosed in the patent to H. S. Black, No. 2,102,67L-December 21, 1937, which is connected into a transmission line carrying signal and pilot currents. The regulator circuit which is connected .to the signal line beyond the amplifier therein is provided with a lter for selecting the pilot current and means for amplifying and rectifying the selected pilot current. The rectified pilotis used to control the oscillatorwhich inf turn controls a resistance element in the feedbackfor beta circuit of the line ampliiier.

The cathode of .the pentode oscillator is heated from a source of alternating current, a rectifier f and a smoothing lter are connected to the source of alternating current for supplying potential to the anode `of theoscillator and a potential regulator is connected to the output ofthe rectifier for supplying constant potential `to the screen grid circuit of the oscillator. The screen grid circuit is controlled in a 'manner to compensate for any cha-nge the oscillator operation caused by variations'in the heating circuit for the oscillator cathode.

It has been found that variations yin 'the potential employed to heat the cathode vof 4the oscillator cause variations i-nthe oscillator output and accordingly variations in the regulating operation.`V yIn order to compensate for the variations in the oscillator output caused by variations in the heating of thejcathode, lthe screen grid circuit is supplied Witha constant potential and the potential yimpres's'efd-Con the .screen Ygrid itself' is controlled according to variations in the potential 'of thehe'ater circuitforthe cathode.

The anodeof the pento'de `.oscillator .is supp-lied with potentialfromaisource of alternating current by means fof fa 'rectifier :of the selenium type and a smoothing filter. vThe source ofialternating current is also connected 'to .the heater for the cathode and any'fchange in .the potential supplied to fthe' cathode .heater is compensated for by changes in. the :potential impressed upon the screen .igrid'of .the oscillator. It has been found that small changes in the potential impressed on .thelano'de o'f ithecscillator cause little change in thehoscillator operation. It has also been found that while the regulator maximum output varies somewhat with A.plate or anode voltage, the position of the cut-off for the oscillator remains practicallyziixed. if the screen grid voltage on the pentode oscillator is heldV constant. In order :to supply s'u'bstantially `constant voltage to Athe screen grid lof the oscillator, `twofcold cathode tubes conn'e'cted inf series with 4a resistance element are shunted across .the output from the selenium rectifier. VA leak resistance is connected across one ofthe cold-cathodel tubes'inorder to initially impressall the'voltage'from the rectifier on the other cold cathode tube 'and' thus insure ignition thereof. Upon ignitionof Aone of the cold cathode tubes, the other tube will be ignited by the ripple voltage `from the rectifier; There is alwaysr substantial ripple voltage from .the rectifier to insure'the ignition of `both cold cathode tubes. Regulation by the two lcold cathode tubes is effected by reason of the resistance-element in series therewith. The screen -gridl circuit is -connectedacross fthe coldoathodel tubesn in circuit withiasmoothingffilter. A

A :resistance'element is inserted in the screen grid circuit-of the .pentode oscillator jto compensate for variations in the oscillator operatic-n caused -byy variations in "thepotential supplied .to

the heater' for the cathode of the oscillator. If the heater current of the oscillator cathode .tends to go down, the potential impressed on the screen grid will be raised by reason of the reduced loss across the resistance element in the screen grid circuit. An increase in the screen grid voltage increases the oscillator output to compensate for the reduction tending to be produced by the reduction in the heater current supplied to .the cathode heater. This operation may take place by reason of supplying the screen grid circuit with substantially constant potential.

The single figure in the accompanying drawing is a diagrammatic View of a regulator circuit constructed in accordance with the invention.

Referring to the drawing, an amplier I of the feedback type is shown in a transmission line of the coaxial type. A transformer 2 connects the input terminals of the amplifier to the coaxial cable and a transformer 3 connects the output terminals of the 'ampliiier to the coaxial cable. The amplifier I may be of the type disclosed in the patent to H. S. Black, No. 2,102,671, December 21, 1937. The amplifier I is shown comprising three tubes 4, 5 and 6 which may be of any suitable type.

An impedance pad 1 is shown connected across the beta circuit 8 of the amplifier I. One element of the impedance pad comprises a resistance element 9 having a high negative temperature coefficient of resistance. The resistance element may bc composed of nickel, manganese and copper oxide or may be composed of silver sulphide. The resistance element 8 is controlled by a heater element I0. An increase in the current flow through the heater element IIJ decreases the resistance of the resistance element 9 to decrease the feedback through the beta circuit 8. This action increases the gain of the amplifier. A decrease in the current iiow through the heater coil I0 produces an opposite action.

Regulator circuits II are .provided for controlling the heater element I0 to maintain the pilot output from the amplifier I substantially constant. The coaxial cable shown on the drawing transmits not only carrier currents but also a pilot current. The pilot current is employed for controlling the operation of the regulator or gain control circuits II. 'Ihe control circuits II comprise a filter I2 which is preferably of the crystal type and which selects the pilot frequency. The crystal filter. I2 is connected directly across the secondary winding of the output transformer 3 connected to the amplifier I. The crystal iilter I2 is connected by a transformer I3 to the input circuit of a pentode amplifier tube I4. The transformer I3 is provided with tuned primary and secondary windings. The windings of the transformer I3 are tuned to the pilot frequency selected by the crystal filter I2. rEhe pentode amplifier I4 comprises a cathode I5 having a heater I6, a suppressor grid I1, a control grid I8, a screen grid I9 and an anode 20. A resistance element 2| provides terminating impedance for the secondary winding of the transformer I3 and a resistance 22 provides terminating impedance for the crystal filter I2. A condenser 23 comprises a part of the tuned primary circuit of the transformer I3.

A transformer 24 having a primary winding 25, a secondary winding 26 and an auxiliary winding 21 connects the output circuit from the pentode tube I4 to a diode rectifier 28. The diode 28 is operated in a circuit of suiiiciently high resistance and with a sumciently high voltage applied to its anode 29 that variations in the heater current for the cathode 3D do not appreciably affect its direct current output. The diode rectiiier 28 comprises an anode 29 and a cathode 30. The auxiliary winding 21 of the transformer 24 is connected through an adjustable condenser 3l to the control grid I8 of the pentode amplifier I4 for neutralization purposes. The secondary winding 26 of the transformer 24 is tuned by an adjustable condenser 32 to the pilot frequency.

Plate potential for the anode 20 of the pentode amplifier I4 is supplied from a source of alterhating current 33 through a selenium rectifier bridge 34 and a smoothing filter 35. The source of alternating current 33 is connected by a transformer 36 to the input vertices of selenium bridge 34. The smoothing iilter connected to the output vertices of the bridge 34 comprises inductance elements 31 and 38 and capacitance elements 39, 40 and 4I. The output terminals of the smoothing filter 35 are connected in circuit with two resistance elements 42` and 43. Potential for energizing the anode 28 of the pentode amplifier I4 is obtained by the potential drop across the resistance element 43. Plate circuit for the pentode I4 may be traced from the anode 20 through the primary winding 25 of the transformer 24, resistance element 43 and resistance elements 44 and 45 to the cathode I5. The p0- tential drop across the resistance element 45 serves as a minimum bias on the pentode I4 and the potential drop across the resistance element 44 serves as an adjustable bias for the pentode I4. The condensers 48a and 41a are connected in shunt respectively with resistance 43 and resistances 44 and 45 in series and serve as by-pass condensers for those resistances.

The rectifier circuit through the diode of rectifier 28 may be traced from the anode 29 through the secondary winding 26 of the transformer 24 and resistance elements 46, 41 and 48 to the cathode 36. The condensers 49 and 50 are connected respectively across these resistance elements in series and across resistance element 48 and the lower variable portion of resistance element 41 in series and serve as by-pass condensers for these resistances. The potential drop across the resistance element 48 and a. portion of the resistance element 41 is impressed on the input circuit of a pentode oscillator tube 5I. The pentode oscillator tube 5I comprises an anode 52, a suppressor grid 53, a screen grid 54, a control grid 55 and a cathode 56 having a heater 51. The heater 51 for the cathode 56 the same as the heater for the cathode 30 in the rectifier 2B and the heater I6 for the cathode I5 in the pentode amplifier I4 is energized from the source of alternating current 33. The output circuit of the pentode oscillator tube 5I is connected by a transformer 58 having a tuned primary winding 59 and a secondary winding 60 to the heater coil I0. Anode potential for the pentode oscillator 5I is obtained directly from the output terminals of the smoothing filter 35 connected to the selenium rectiiier 34.

The screen grid circuit for the pentode oscillator 5I is connected by a regulator 6I to the rectiiier 34 ahead of the smoothing iilter 35. The regulator 6I comprises two cold cathode tubes 62 and 63 and a resistance element 64. The cold cathode tubes 62 and 63 are connected in series with the resistance element 64 across the output terminals of the selenium rectifier 34. The resistance element 64 serves as a regulating resistance to produce substantially constantvolta'ge across the two tubes 62 `and'63.. A. resistance element 65 is connected across the Acold cathode tube 63v in order to permit ignition of the tube 62 by the entire voltage from the rectifier 34. Ignition of the cold cathode tube 63 isl insured by the ripple voltage from the rectifier output. Assuming that the two tubes 62 and 63- each have a striking voltage of I and an operating voltage of 60 it is clear the two tubes cannot be ignited simultaneously if only 130 volts are supplied from the source. However, by reason of the resistance 65 shunted around the tube 63, the tube 62 may first be ignited and after ignition oftube 62 effect the ignition of the tube 53.` A resistance element 66 and a condenser 61 serve to filter out the ripple voltage appearing across the two cold cathode tubes 62 and 63.

The screen grid circuit for the oscillator pentode I may be traced from the' screen grid 54 through a resistance element 68, resistance element 66 to one electrode of the cold cathode tube 63 and from an electrode of the cold cathode tube 62 to the cathode 56 of the oscillator 5|. The resistance element 68 serves to vary the potential impressed on the screen grid 54 in a manner to compensate for any changes in the potential impressed on the heater 541 for the cathode 56. If the heater potential employed to heat the cathode 56 goes down then there is a tendency to reduce the screen grid current which flows through the resistance element 68. A reduction in the screen grid current reduces the potential drop across the resistance 68 which in turn increases or raises the potential impressed on the screen grid 54. This raising of the screen grid potential compensates for the reduction in the oscillator output which would be caused by the reduction in the heater current supplied to the cathode 5G. If the heater current tends to increase an opposite action takesplace. 'I'he variations inthe gain of the amplifier I.4' caused by changes in the heater current for the cathode I5 and the variations in the potential impressed on the anode 2G and the screen grid I9 are opposite and about equal to the shift in the operating point of the oscillator tube 5I caused by variations in the potential supplied to the anode 52.

The oscillator 5I is of the type disclosed in the R. R. Blair patent, No. 2,178,333, October 31, 1939. The oscillator is adjusted to have an outputinput characteristic curve with a steeply inclined cut-off portion. The oscillator is operated along the steeply inclined cut-off portion so that a small change in bias on the grid 55 will cause a more than proportional change in the output from the oscillator. The bias impressed on the grid 55 of the oscillator 5I varies according tothe strength of the pilot current on the coaxial cable. The pilot current is selected by the crystal filter I2, amplified by the pentode amplifier I4, rectified by the diode 28 and impressed on the grid 55 in order to control the oscillator according to the strength of the pilot current.

The circuit for energizing the heater coil I0 by the oscillator output may be traced from one terminal or" the secondary winding 6I) of the transformer 58 through the coil I0 and resistance element 69 to the other Aterminal of the secondary winding 60.V The potential drop across the resistance element 69 energizes anautotransformer I0 for use as an oscillator feed-backvltage on the oscillator 5 I. The autotransformer I0 is' connected to a resistance element 'II shunted by .condenser 12. The resistance 'II 'is adjusted across the autotransformer vsecondary winding to obtain an exact degree of cut-"oil vsharpness for good regulationwitlrout hunting Yacticu. The adjustable resistance Il is connected to the control grid 55-of1thel oscillator by meansof afcondenser T3'.

'I'he pentode oscillator 5I: has avery" high resistance and serves in fact as a constant current generator; The constant current supplied bythe oscillatorwto the secondary windinglill of lthe,

transformer 58 lfiowsin series through the load resistance comprising the heater coil I0 and the resistance 69 `which controls the- .oscillator feed-n back. The' load resistance comprising the heater coil I0 maybeofthe order of 10`0 ohms and effects little interference with the constant current flowing through the circuit of `the secondary windingy 60. 'Ihus the outputfrom the oscillator 5I is independentof the resistance-of the heaterA coil I0. Any suitable heater coil may be coupled to 4the oscillatoroutput without changing the oscillator operation. The resistance 69 being of the order of 20 ohms suppliesa constant potential to the autotransformer 'I0 by lreason of the constant current iiow therethrough. The autotransform'er 'IIJ serves merely to step-up the potential' drop from across the resistance element 69;v

In a regulator circuit of the above-described type, the pilot'frequency appearing` beyond the` amplifierV I is selected by the vcrystal filter I2, amplified by `the pentodey I4 and rectified by the diode 28 for controlling `the oscillator 5I. The oscillator 5| controls the current iiow through the heater coil I0 for adjusting the feedback on the line amplifier I. When the energy level of pilot current beyond the amplier I falls below -normal value there isa reduction in the negative bias impressed on the control grid 55 of the oscillator 5 I-. This causes a relative increase in theheater current supplied tothe heater coil I0. An increase in the current flow through the heater element increases the temperature o1 the resistance element '9 to decrease the resistance -value thereof. The feedback on the amplifier I is thereby reduced to increase the gain eiected by the amplifier. In case the energy level of the pilot current beyond the varnpliiier I tends to increase above level then an opposite action takes place. In the operation of the oscillator pentode 5I', the cathode 56 `vis heated 'by current froman alterhating current source and provision is made by the regulator 6I and the resistance element 68 to insure that a change in the transconductance of oscillator tube 5I caused by aging of the tube or by a change in the potential on the heater for the cathode 56 of the oscillator will notefect the regulating operation of the oscillator.

Modications in the circuits and in the arrangement and location of parts may be' made within the spirit and scope of the invention andv such modifications' are intended to 'be covered by the appended claims.

What is claimed isz' l. In'a regulator circuit, an oscillator having an output-input curve with a 'steeply inclined cut-off portion, said oscillator comprising a vacuum tube having a cathode, a control grid. a screen grid and an anode, a source of alternating current, means for heating the cathode of said oscillator from' said source of alternating current, means for rectifying current from said source of alternating current, means including .a smoothing means connecting said rectifying means to the anode of said oscillator to supply potential to the oscillator anode, means additional to and exclusive of 'said smoothing means which additional means includes a source of constant voltage and is connected to said rectifying means for supplying potential to the screen grid of said oscillator, and means connecting said source of constant voltage of said additional means to said screen grid circuit for controlling the screen grid circuit of said oscillator independently of the other electrodes by changing the potential of said screen grid to compensate for changes in the oscillator operation caused by changes in the potential from said source of alternating current supplied to heat the oscillator cathode.

2. In a regulator circuit, a pentode tube, a source of alternating current, means for heating the cathode of said pentode tube from the source of alternating current, rectifying means connected to said source of alternating current for supplying rectified potential to the screen grid and the anode of said pentode tube, a smoothing lter for connecting said rectifying means to the pentode anode, means including a source of constant voltage connected to said rectifying means to supply voltage to the screen grid of said pentode independently of said smoothing filter, and means connecting said supply means to said screen grid for controlling the screen grid circuit of said pentode by changing the potential impressed on said screen grid to compensate for changes in the pentode operation caused by changes in the potential from said source of alternating current supplied for heating the pentode cathode.

3. In a regulator circuit, a pentode tube, a source of alternating current, rectifying means connected to said source of alternating current for supplying rectified potential to the screen grid and the anode of said pentode tube, `a smoothing filter for connecting said rectifying means to the pentode anode, two cold cathode tubes connected in series with a resistance element across said rectifying means and before said filter to insure ignition of -said cold cathode tubes by the ripple voltage, a resistance in shunt with one of said cold cathode tubes for insuring ignition of said tubes in succession, and means for connecting the screen grid circuit of said pentode tube across said cold cathode tubes.

4. In a regulator circuit, a pentode oscillator having an output-input characteristic curve with a steeply inclined cut-off portion, a source of rectified potential, a smoothing filter connecting said source to the anode of said pentode, two cold cathode tubes connected in series with a resistance element across the source of rectiiied current before said lter to insure ignition of said tubes by the ripple voltage, a resistance in shunt with one of said tubes for insuring ignition of said tubes in succession, and means for connecting the screen grid circuit of said pentode across said cold cathode tubes.

5. In a regulator circuit, a pentode oscillator having an output-input characteristic curve with a steeply inclined cut-oit portion, a source of alternating current, means for heating the cathode of said oscillator from said source, means for rectifying current from said source, a smoothing filter connected to said rectifying means, means to supply potential from said smoothing lter to said oscillator anode, two cold cathode tubes connected in series with a resistance element across said rectifying means ahead of said smoothing filter to eiiect ignition of said tubes by the ripple voltage from the rectifying means, a resistance in shunt with one of said tubes for insuring ignition of said tubes in succession, a

circuit for connecting thescreen grid circuit of said oscillator across'said tubes and a resistance element inserted in said screen circuit to compensate for changes in the oscillator operation caused by changes in the potential from said source of alternating current supplied to the heater for the oscillator cathode.

6. In a regulator circuit, a pentode oscillator having an output-input characteristic curve with a steeply inclined cut-ofi portion, a source of alternating current, means for heating the cathode of said pentode tube from the source o! alternating current, rectifying means connected to said source of alternating current for supplying rectiiied potential to the screen grid and the anode of said pentode tube, a smoothing lter for connecting said rectifying means to the pentode anode, two cold cathode tubes connected in series with a resistance element across said rectifying means and before said iilter to insure ignition of said cold cathode tubes by the ripple voltage, a resistance in shunt with one of said cold cathode tubes for insuring ignition of said cold cathode tubes in succession, means for connecting the screen grid circuit of said pentode tube across said cold cathode tubes to energize the screen grid circuit, and means for controlling the screen grid circuit of said pentode to compensate for changes in the potential from said source of alternating current supplied to heat the pentode cathode.

'7. In a regulator circuit, a pentode tube, a source of alternating current, rectifying means connected to said source of alternating current for supplying rectified potential to the screen grid and the anode of said pentode tube, a smoothing iilter for connecting said rectifying means to the pentode anode, two cold cathode tubes connected in series with a resistance element across said rectifying means and before said filter to insure ignition of said cold cathode tubes by the ripple voltage, the voltage across said two tubes in series required to ignite them being greater than the voltage generated by said rectifier, a resistance in shunt with one of said cold cathode tubes only for insuring ignition of said tubes in succession, and means for connecting the screen grid circuit of said pentode tube across said cold cathode tubes.

8. In a regulator circuit, a pentode oscillator having an output-input characteristic curve with a steeply inclined cut-off portion, a source of rectiiied potential, a smoothing filter connecting said source to the anode of said pentode, two cold cathode tubes connected in series with a resistance element across the source of rectified current before said filter to insure ignition of said tubes by the ripple voltage, the voltage across said two tubes in series required to ignite them being greater than the voltage generated by said rectiiier, a resistance in shunt with one of said tubes only for insuring ignition of said tubes in succession, and means for connecting the screen grid circuit of said pentode across said cold cathode tubes.

9. In a regulator circuit, a pentode oscillator having an output-input characteristic curve with a steeply inclined cut-oif portion, a source of alternating current, means for heating the cathode of said oscillator from said source, means for rectifying current from said source, a smoothing filter connected to said rectifying means, means to supply potential from said smoothing iilter to said oscillator anode, two cold cathode tubes connected in series with a resistance element across said rectifying means ahead of said smoothing filter to effect ignition of said tubes by the ripple voltage from the rectifying means, the voltage across said two tubes in series required to ignite them being greater than the voltage generated by said rectifier, a resistance in shunt with one of said tubes only for insuring ignition of said tubes in succession, a circuit for connecting the screen grid circuit of said oscillator across said tubes and a resistance element inserted in said screen circuit to compensate for changes in the oscillator operation caused by changes in the potential from said source of alternating current supplied to the heater for the oscillator cathode.

10. In a regulator circuit, a pentode oscillator having an output-input characteristic curve with a steeply inclined cut-off portion, a source of alternating current, means for heating the cathode of said pentode tube from said source of alternating current, rectifying means connected to said source of alternating current for supplying rectiiied potential to the screen grid and the anode of said pentode tube, a smoothing filter connecting said rectifying means to the pentode anode, two cold cathode tubes connected in series with a resistance element across said rectifying means and before said iilter to insure ignition of said cold cathode tubes by the ripple voltage, the voltage across said two tubes in series required to ignite them being greater than the voltage generated by said rectifier, a resistance in shunt with one only of said cold cathode tubes for insuring ignition of said cold cathode tubes in succession, and means for connecting the screen grid circuit of said pentode tube across said cold cathode tubes to energize the screen grid circuit, including means fol` controlling the screen grid circuit of said pentode to compensate for changes in the output of the pentode oscillator due to changes in the potential from said source of alternating current supplied to heat the pentode cathode.

11. In a regulator circuit, a pentode tube, a source of alternating current, means for heating the cathode of said pentode tube from said source of alternating current, rectifying means connected to said source of alternating current for supplying rectified potential to the screen grid and the anode of said pentode tube, a smoothing filter connecting said rectifying means to the pentode anode, constant voltage regulating means comprising a cold cathode tube connected to said rectifying means independently of said smoothing iilter, and means connecting said constant voltage means to the screen grid of said pentode for controlling the screen grid circuit of said pentode by changing the potential impressed on said screen grid to compensate for changes in the pentode operation caused by changes in the potential from said source of alternating current supplied for heating the pentode cathode.

y 12. In a regulator circuit, a pentode tube, a source of alternating current, rectifying means connected to said source of alternating current for supplying rectied potential to the screen grid and the anode of said pentode tube. a smoothing filter connecting said rectifying means to the pentode anode, two cold cathode tubes connected in series with a resistance element across said rectifying means and before said Iilter to insure ignition of said cold cathode tubes by the ripple voltage, a resistance of proper amount connected in shunt with one of said cold cathode tubes to make the conductances between the terminals of said cold cathode tubes individually different for insuring ignition of said tubes in succession, and means for connecting the screen grid circuit of said pentode tube across said cold cathode tubes.

13. In a regulator circuit a vacuum tube having a cathode, a control grid, a screen grid and an anode, a source of alternating current, means for heating the cathode of said vacuum tube from said source of alternating current, rectifying means connected to said source of alternating current for supplying potential to the screen grid and the anode of said vacuum tube, a smoothing iilter connected to said rectifier, means connecting the output of said smoothing filter to the anode of said vacuum tube, means connected to the rectifier ahead of said smoothing lter producing a substantially constant voltage, and means comprising a resistance connecting said constant voltage means to said screen grid whereby changes in the output of the vacuum tube caused by changes in the potential from said source of alternating current supplied for heating the cathode of said vacuum tube are compensated.

14. In a regulator circuit, a vacuum tube having a cathode, a control grid, a screen grid and an anode, a source of alternating current, means for heating the cathode of said vacuum tube from said source of alternating current, rectifying means connected to said source of alternating current for supplying rectified potential to the screen grid and the anode of said vacuum tube, a smoothing filter connecting said rectifying means to the anode of said vacuum tube, constant voltage regulating means comprising a cold cathode tube connected to said rectifying means independently of said smoothing nlter, and means connecting said constant Voltage means to the screen grid of said vacuum tube for controlling the screen grid circuit of said vacuum tube by changing the potential impressed on said screen grid to compensate for changes in the operation of said vacuum tube caused by changes in the potential from said source of alternating current supplied for heating the cathode of said vacuum tube.

ROYER R. BLAIR. 

